The present invention relates to an image formation apparatus, and an image formation method using the same. The image formation apparatus may be any of photocopiers, printers, facsimiles, and composite machines based on the electrophotographic technique. More particularly, the present invention relates to a non-contact development-type image formation apparatus with a two-component developer containing magnetic carrier particles and nonmagnetic toner, and an image formation method using the same. In this image formation apparatus, the nonmagnetic toner is charged by the magnetic carrier. Only the charged toner is kept on a developer roller and is flown onto an electrostatic latent image for development.
There are known image formation apparatuses with the two-component developer, so-called hybrid development apparatus, as disclosed in, for example, Japanese Published Unexamined Patent Application (Kokai Tokkyo Koho) no. 6-67546 and No. 7-92804. This development apparatus has a developer roller opposed to an image holder having an electrostatic latent image and a magnetic roller which is put on the opposite place of the image holder on the developer roller, detached from the developer roller. Applicability of such image formation apparatuses to one-drum color overlay technique, which successively forms multiple image formation portions comprising a charging unit and an exposure optical unit and so on corresponding to multiple color image on a photoreceptor, has been examined with the recent demand for high-speed image processing. According to this image formation apparatus, it is possible to form a color image with no significant color shift because of accurate overlay of multiple toners on the photoreceptor. Therefore, the potential of this technique for the high-quality color imaging has been thus been noted. Further, this color image forming apparatus is noted since said two-component development system does not disturb a toner image that is formed by an image forming portion for other colors.
Another proposed technique for high-speed color imaging is a tandem system that uses multiple photoreceptors according to respective color toners and successively forms respective color component images in an overlapping manner on a transfer medium in synchronism with a feed of the transfer medium. In the tandem system, said two-component development system is noted as a non-contacting developing system which does not afford excessive stress to a photoreceptor drum.
In the known two-component development system, however, the presence of non-developed fine particles and additives in the developer lowers the development performance. Narrowing the gap between a developer roller and a photoreceptor drum and raising an AC peak voltage (Vpp) are possible measures to enhance the development performance. These measures, however, cause leakage of an applied potential between the developer roller and the photoreceptor drum. The leakage of the applied potential adversely affects the development system that aims to shift only the sufficiently charged toner of the two-component developer onto the developer roller while keeping the carrier particles unmoved.
The object of the present invention is accordingly provide an image formation apparatus and an image formation method using the same, that effectively prevent leakage of an applied voltage between a developer roller and a photoreceptor drum in the process of developing an electrostatic latent image on the photoreceptor by means of a thin toner layer, which is formed on the surface of the developer roller by a magnetic brush comprising toner and carrier particles on the magnetic roller, so as to form an image.